Lead-free optical light flint glass materials

ABSTRACT

The lead-free optical light flint glass material has a refractive index n d  between 1.54 and 1.61 and an Abbe number ν d  of between 38 and 45. It has a composition, based on oxide content, of SiO 2  from 52 to 62% by weight, Al 2 O 3  from 3 to 8% by weight, Na 2 O from 7 to 14% by weight, K 2 O from 8 to 14% by weight, TiO 2  from 13 to 18% by weight, ZrO 2  from 0 to 5% by weight, with at least one refining agent, if needed, in an amount suitable for the purposes of refining; and with the proviso that a sum total of Na 2 O and K 2 O present is greater than 18% by weight.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to lead-free optical glass materials which have refractive indices n_(d) of between 1.54 and 1.61 and Abbe numbers ν_(d) of between 38 and 45. Glass materials of this type belong to the optical glass family of the light flint glass materials.

2. Prior Art

Because the glass ingredients PbO and As₂O₃ have become the subject of public discussions regarding environmental pollution which they cause, the manufacturers of optical equipment need PbO-free and preferably also As₂O₃-free glass materials having the appropriate optical properties.

Simply exchanging the lead oxide for one or more ingredients does not generally allow the PbO-influenced, desired optical and glass technology properties to be reproduced. Instead, new developments or far-reaching changes in the glass composition are required.

Lead-free glass materials with similar optical characteristics and a similar composition are already known.

The closest prior art glass materials are described in JP 56-59640 A. However, the glass materials for spectacles described in that document contain only 11 to 18% by weight alkali metal oxides, which may be composed of Na₂O, K₂O and Li₂O. Furthermore, they optionally contain the ingredients Nb₂O₅, La₂O₃ and also ZrO₂, which in this case inhibit crystallization but are expensive, as partial replacements for TiO₂.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a lead-free optical glass material with a refractive index n_(d) of between 1.54 and 1.61 and an Abbe number ν_(d) of between 38 and 45 which can be produced at low cost and have good melting and processing properties. This also includes the requirement for sufficient crystallization stability.

According to the invention the lead-free optical glass material with a refractive index n_(d) of between 1.54 and 1.61 and an Abbe number ν_(d) of between 38 and 45 has a composition, based on oxide content, of:

SiO₂ from 52 to 62% by weight, Al₂O₃ from 3 to 8% by weight, Na₂O from 7 to 14% by weight, K₂O from 8 to 14% by weight, TiO₂ from 13 to 18% by weight, ZrO₂ from 0 to 5% by weight,

and at least one refining agent, if needed, in an amount suitable for the purposes of refining; with the proviso that a sum total of Na₂O and K₂O present is greater than 18% by weight.

The glass materials according to the invention contain the glass formers SiO₂ and Al₂O₃, in balanced proportions of 52 to 62% by weight SiO₂ and 3 to 8% by weight Al₂O₃. In this way, both the meltability of the glass materials, which deteriorates as the Al₂O₃ content rises, and their chemical durability, which would deteriorate if the Al₂O₃ content were too low, are satisfactory. Moreover, if the above-mentioned minimum Al₂O₃ content were to be increased, the risk of devitrification would rise excessively. There is no B₂O₃, which is a common third glass former, used, which is of benefit to the chemical durability of the glass materials. An SiO₂ content of between 53 and 59% by weight is preferred, but an SiO₂ content of between 55 and 59% is particularly preferred. The Al₂O₃ content is preferably between 4 and 7% by weight, particularly preferably between 5 and 7% by weight.

To obtain the desired high refractive index combined, at the same time, with a low Abbe number, the glass materials contain relatively large amounts of TiO₂, at 13 to 18% by weight. The TiO₂ content is preferably between 14 and 17% by weight, particularly preferably between 15 and 17% by weight. In addition to TiO₂, the glass materials according to the invention may also contain up to 5% by weight ZrO₂. The presence of the two ingredients improves the chemical durability, in particular the alkali resistance. If the levels were to be higher, the crystallization stability would be considerably reduced. Preferably, the sum of the TiO₂ and ZrO₂ is less than 18% by weight. In preferred embodiments, there is no ZrO₂.

To improve the meltability, that is to reduce the melting temperatures, with the given high level of glass formers, the glass materials contain from 7 to 14% by weight Na₂O and 8 to 14% by weight K₂O as fluxing agents. If the levels were to be higher, in particular in relation to the high TiO₂ content, the tendency towards crystallization would rise excessively. Therefore, the sum of R₂O and TiO₂ is preferably less than or equal to 42% by weight. For the same reason, LiO₂ is dispensed with altogether. Increasing the alkali metal content further would also mean that the desired range of refractive indices would not be reached. The sum of Na₂O and K₂O is at least greater than 18% by weight. An Na₂O content of 8 to 13% by weight and a K₂O content of 10 to 14% by weight are preferred. An Na₂O content of 8 to 11% by weight and a K₂O content 10 to 12% by weight are particular preferred.

To improve the quality of the glass, one or more refining agents, which are known per se, may be added in the customary amounts to the batch in order to refine the glass material. This provides the glass material with a particularly good internal glass quality with regard to the freedom from bubbles and cords.

If the refining agent used is not As₂O₃, but rather, instead, Sb₂O₃, for example, which is possible without any loss in glass quality, the lead-free glass materials according to the invention are additionally free of arsenic.

The Sb₂O₃ content is preferably between 0.1 and 0.5% by weight. It is also preferable for the glass materials, if appropriate in addition to Sb₂O₃, to contain up to 0.5% by weight fluoride, which likewise has a refining action. Fluoride is added, for example, as NaF.

In addition to a refractive index n_(d) of between 1.54 and 1.61, and an Abbe number of between 38 and 45, and a positive anomalous partial dispersion in the blue region of the spectrum, the glass materials according to the invention have the benefits as described in the following paragraphs.

The glass materials are PbO-free and, in a preferred embodiment are also As₂O₃-free. The glass materials exhibit good crystallization stability. This allows production in a continuously operating melting unit. A measure of the crystallization stability being sufficient for production of this nature is the viscosity at the liquidus temperature. For continuous production, it should be ≧1000 dPas. This condition is satisfied with the glass materials according to the invention. Since the glass materials exhibit a crystallization stability of this nature, further thermal treatment of the glass materials, such as pressing or repressing, also becomes possible. It is advantageous that it is possible to dispense with the expensive ingredients Nb₂O₅ and La₂O₃ while the glass materials, despite the high TiO₂ content, still exhibit a sufficient crystallization stability, this fact also being contributed to by the fact that alkaline-earth metal oxides are eliminated altogether. Therefore, apart from inevitable impurities, the glass materials are free of alkaline-earth metal oxides, Nb₂O₅ and La₂O₅.

In addition, the “length of the glass materials” (viscosity curve which falls slowly with temperature) ensures good processability despite the relatively high melting temperatures of approximately 1400° C.

The glass materials have an exceptional alkali resistance, as attested to by the fact that they belong to the alkali resistance class AR=1, while also retaining a chemical durability which is adequate in other respects. The chemical durability of the glass materials is of importance for their further processing, such as grinding and polishing.

Glass materials within a compositional range (in % by weight based on oxides) SiO₂ from 52 to 62, preferably from 53 to 59, % by weight; Al₂O₃ from 3 to 8, preferably from 4 to 7, % by weight; Na₂O, from 7 to 14% by weight, K₂O from 8 to 14% by weight, with a sum of Na₂O and K₂O greater than 18% by weight; preferably Na₂O from 8 to 13% by weight and K₂O from 10 to 14% by weight; TiO₂ from 13 to 18, preferably from 14 to 17, % by weight; ZrO₂ from 0 to 5% by weight, with a sum total of TiO₂ and ZrO₂ present is not greater than 18% by weight, preferably ZrO₂-free; have a refractive index n_(d) of between 1.55 and 1.60 and an Abbe number ν_(d) of between 39 and 44. Glass materials of composition (% by weight based on oxides present) SiO₂ from 55 to 59% by weight; Al₂O₃ from 5 to 7% by weight; Na₂O, from 8 to 11% by weight; K₂O, from 10 to 12% by weight, with a sum of Na₂O+K₂O greater than 18% by weight, TiO₂ from 15 to 17% by weight have a refractive index n_(d) of between 1.56 and 1.59 and an Abbe number ν_(d) of between 39 and 44.

EXAMPLES

Six examples of glass materials according to the invention were melted from standard raw materials.

Table 2 lists the respective composition (in % by weight based on oxides), the refractive index n_(d), the Abbe number ν_(d), the partial dispersion in the blue region of the spectrum P_(g,F) and the anomaly of this partial dispersion ΔP_(g,F) [10⁻⁴], the density ρ[g/cm³], the coefficient of thermal expansion α_(20/300) [10⁻⁶/K] and the transformation temperature T_(g) [° C.] for the glass materials.

The glass materials according to the invention were produced as follows:

The raw materials for the oxides, preferably carbonates and nitrates, were premixed, the refining agent was added and then thorough mixing took place. The glass batch was melted in a discontinuous melting unit at melting temperatures of between approximately 1400° C. and 1450° C., then refined and extensively homogenized. The casting temperature was approximately 1300° C.

Table 1 shows a melting example.

TABLE 1 Example of how to melt 100 kg of calculated glass Weighed-in % by Raw quantity Oxide weight material (kg) SiO₂ 51.7 SiO₂ 51.72 Al₂O₃ 8.0 Al(OH)₃ 12.35 Na₂O 8.0 Na₂CO₃ 13.70 K₂O 14.0 K₂CO₃ 20.59 TiO₂ 17.0 TiO₂ 17.10 ZrO₂ 1.0 ZrO₂ 1.02 Sb₂O₃ 0.3 Sb₂O₃ 0.30

The properties of the glass material obtained in this way are given in Table 2, Example 2.

TABLE 2 Composition (in % by weight based on oxide) of glass materials according to the invention, and significant properties of the glass materials 1 2 3 4 5 6 SiO₂ 52.7 51.7 54.7 62.0 58.7 54.7 Al₂O₃ 7.0 8.0 4.0 3.0 5.0 7.0 Na₂O 13.0 8.0 13.0 14.0 9.0 11.0 K₂O 11.0 14.0 14.0 8.0 10.0 12.0 TiO₂ 16.0 17.0 14.0 13.0 17.0 15.0 ZrO₂ 1.0 Sb₂O₃ 0.3 0.3 0.3 0.3 0.3 n_(d) 1.5829 1.5906 1.5705 1.5663 1.58763 1.57648 V_(d) 41.44 39.81 43.82 43.88 39.67 42.33 P_(g,F) 0.5764 0.5797 0.5713 0.5709 0.5804 0.5737 Δ P_(g,F) [10⁻⁴] 23 29 12 9 33 11 ρ[g/cm³] 2.61 2.61 2.60 2.57 2.59 2.59 α_(20/300) [10⁻⁶/K] 11.2 10.2 12.2 10.4 9.2 10.7 T_(g) [° C.] 558 575 525 541 582 557 

We claim:
 1. A lead-free optical glass material with a refractive index n_(d) of between 1.54 and 1.61 and an Abbe number ν_(d) of between 38 and 45, said glass material consisting, based on oxide content, of SiO₂ from 52 to 62% by weight, Al₂O₃ from 3 to 8% by weight, Na₂O from 7 to 14% by weight, K₂O from 8 to 14% by weight, TiO₂ from 13 to 18% by weight, ZrO₂ from 0 to 5% by weight,

and from 0.1 to 0.5% by weight of Sb₂O₃ as refining agent; and with the proviso that a sum total of said Na₂O and said K₂O present is greater than 18% by weight.
 2. A lead-free optical glass material with a refractive index n_(d) of between 1.55 and 1.60 and an Abbe number ν_(d) of between 39 and 44, said glass material consisting, based on oxide content, of SiO₂ from 52 to 62% by weight, Al₂O₃ from 3 to 8% by weight, Na₂O from 7 to 14% by weight, K₂O from 8 to 14% by weight, TiO₂ from 13 to 18% by weight, ZrO₂ from 0 to 5% by weight,

and from 0.1 to 0.5% by weight of Sb₂O₃ as refining agent; with the proviso that a sum total of said Na₂O and said K₂O present is greater than 18% by weight; and with the proviso that a sum total of said TiO₂ and said ZrO₂ present is not greater than 18% by weight.
 3. A lead-free optical glass material with a refractive index n_(d) of between 1.55 and 1.60 and an Abbe number ν_(d) of between 39 and 44, said glass material consisting, based on oxide content, of SiO₂ from 52 to 62% by weight, Al₂O₃ from 3 to 8% by weight, Na₂O from 7 to 14% by weight, K₂O from 8 to 14% by weight, TiO₂ from 13 to 18% by weight, ZrO₂ from 0 to 5% by weight,

and from 0.1 to 0.5% by weight of Sb₂O₃ and at least one fluoride in an amount of up to 5% by weight as refining agent; and with the proviso that a sum total of said Na₂O and said K₂O present is greater than 18% by weight.
 4. A lead-free optical glass material with a refractive index n_(d) of between 1.55 and 1.60 and an Abbe number ν_(d) of between 39 and 44, said glass material consisting, based on oxide content of SiO₂ from 53 to 69% by weight, Al₂O₃ from 4 to 7% by weight, Na₂O from 8 to 13% by weight, K₂O from 10 to 14% by weight, TiO₂ from 14 to 17% by weight,

and at least one refining agent, if needed, in an amount suitable for the purposes of refining; and with the proviso that a sum total of said Na₂O and said K₂O present is greater than 18% by weight.
 5. A lead-free optical glass material with a refractive index n_(d) of between 1.56 and 1.59 and an Abbe number ν_(d) of between 39 and 44, said glass material consisting, based on oxide content, of SiO₂ from 55 to 59% by weight, Al₂O₃ from 5 to 7% by weight, Na₂O from 8 to 11% by weight, K₂O from 10 to 12% by weight, TiO₂ from 15 to 17% by weight,

and at least one refining agent, if needed, in an amount suitable for the purposes of refining; and with the proviso that a sum total of said Na₂O and said K₂O present is greater than 18% by weight.
 6. A lead-free optical glass material as defined in claim 4 or 5, wherein said at least one refining agent comprises Sb₂O₃ and said amount of said Sb₂O₃ is from 0.1 to 0.5% by weight.
 7. A lead-free optical glass material as defined in claim 4 or 5, wherein said at least one refining agent comprises at least one fluoride and up to 0.5 percent by weight of said at least one fluoride is present.
 8. A lead-free optical glass material as defined in claim 4 or 5, and substantially free of arsenic oxide above a level of inevitable impurities. 